Hypsometers were originally used for determining atmospheric pressure by measuring the boiling point of a liquid. The boiling points of most fluids are functions of pressure, and for some liquids (such as water), the relationship between boiling point and pressure has been extensively studied and the results published. Thus if one wants to know the atmospheric pressure, one can determine it by measuring the boiling point of a fluid (such as water) for which the relationship between boiling point and pressure is known. Conversely, if the atmospheric pressure is accurately known, then a hypsometer can be used to calibrate thermometers at the boiling point of a fluid such as water. The present invention is concerned with the use of hypsometers as a means for calibrating thermometers at a boiling point, more specifically, the boiling point of pure water.
Calibration of precision thermometers requires the use of accurately known temperature standards, probably the best known standard is the triple point of water. However the triple point provides a standard at only one temperature. Often, calibrations at other temperatures are needed. The boiling point of water is a convenient standard at around 100 degrees C. It is inherently not as reliable as the triple point because the boiling point depends on the atmospheric pressure, but assuming that the atmospheric pressure can be measured independently to a high enough degree of accuracy, then the boiling point of fluids (such as water) having accurately known relationship between boiling and pressure is useful for calibration of thermometers.
The present invention provides a hypsometer for calibration of thermometers at the boiling point of water to an accuracy of 0.01 degree C.
One preferred embodiment of the invention provides a hypsometer for use to calibrate thermometers comprising 1) an elongated vessel for containment of a working fluid, 2) means for heating said fluid to its boiling point, 3) a vapor jacket to provide a path for the vapor to escape to the atmosphere, 4) a vapor chamber near the lower end of the vapor jacket, and 5) a thermometer holder to hold a thermometer at a position with it sensor tip inside the vapor chamber. Equalization of pressure between the interior of the vessel and the outside atmosphere is assured by allowing the vapor to escape through the vapor jacket. Equalization of pressure is further assisted by having the vessel long enough to assure that as the fluid is heated to boiling at one end of the vessel, vapor condenses near the other end at a sufficient rate to prevent any appreciable increase of pressure inside the vessel. The vapor jacket also serves as a means for liquid to enter the vessel; the liquid can be condensed vapor or working fluid from an external source added to the vessel for replenishment of the supply of working fluid in the vessel; Near the lower end of the vapor jacket is a narrow opening where working fluid, in liquid state, collects to form a liquid seal allowing liquid to pass through (downward), but preventing passage of vapor (upward), thereby assuring that the vapor from the vessel must pass through the vapor chamber (around the sensor tip of the thermometer) before escaping into the atmosphere.